A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or a hydrocarbon (e.g., methanol), to electrical power through an electrochemical process rather than by combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as proper reactants are supplied from an outside source.
With respect to state-of-the-art fuel cell systems generally, several different configurations and structures have been contemplated. In this regard, existing fuel cell systems are typically classified based on one or more criteria, such as, for example: (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, and (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly. In general, however, it is perhaps most customary to classify existing fuel cell systems by the type of electrolyte (i.e., ion conducting media) employed within the electrode stack assembly. Accordingly, most state-of-the-art fuel cell systems have been classified into one of the following known groups:
1. Phosphoric acid fuel cells (e.g., phosphoric acid electrolyte);
2. Alkaline fuel cells (e.g., KOH electrolyte);
3. Molten carbonate fuel cells (e.g., Li2CO3/K2CO3 electrolyte);
4. Solid oxide fuel cells (e.g., yttria-stabilized zirconia electrolyte);
5. Proton exchange membrane fuel cells (e.g., NAFION electrolyte).
Unfortunately, existing state-of-the-art fuel cell systems are not entirely satisfactory for the production of small-scale portable direct feed fuel cell systems, in part, because of problems associated with achieving a small form factor while maintaining a relatively high power density. In addition, existing fuel cell technology has not addressed many of the concomitant problems associated with closed circulating microfluidic liquid feed fuel cell systems adapted for use with portable electronic devices. More specifically, and at the present time, there has been very little in way of research and development directed toward nitric acid regeneration fuel cell systems having a recirculating nitric acid catholyte flow stream. Previous known fuel cell systems that have contemplated the use and regeneration of a nitric acid catholyte flow stream include, for example, those systems disclosed in U.S. Pat. No. 3,261,717, U.S. Pat. No. 3,281,274, U.S. Pat. No. 3,318,735, U.S. Pat. No. 3,350,227, U.S. Pat. No. 3,979,225, and U.S. Pat. No. 4,528,250. Most all of these systems, however, use a nitric acid catholyte flow stream that is regenerated with oxygen gas from the air; and as such, these fuel cell systems are not optimal for working within a closed recirculating microfluidic liquid feed fuel cell system adapted for use with a portable electronic device.
Accordingly, there is still a need in the art for new and improved nitric acid regeneration fuel cell systems. More specifically, there is a need for nitric acid regeneration fuel cell systems having a recirculating nitric acid catholyte flow stream that is regenerated with a secondary oxidant flow stream, and that incorporates microfluidic and microelectromechanical systems (“MEMS”) technologies so as to achieve a high power density within a small form factor. The present invention fulfills these needs and provides for further related advantages.